I’ve just come across this article. I know about that guy – he has been flying a TB20 for many years.
It seems he over-primed, ended up with some fuel going as far as the air filter, which then caught fire.
But I wonder how much over-priming you would need to do for that to happen?
That illustration in the article, showing the little red lever, I don’t recognise. Maybe older TB20s had that device.
But I wonder how much over-priming you would need to do for that to happen?
Normally, overpriming alone should not cause this case of fire. On all engine installations I have come across so far, there is a vent somewhere that will drain excess fuel out of the engine compartment before it can reach the air filter or other sensible/inflammable parts. In fact, “my way” of starting (or getting them started by students) cold fuel injected piston engines is to prime very generously, unless some precise figure is given like in the TB20 case. In winter, you often have only enough battery power for two or three starting attempts, so prime as if there is no tomorrow… On some piston twins like the Cessna 400 series, you prime a cold engine until you can see the fuel run out below the engine cowlings.
It is sometimes very difficult to determine whether an engine wouldn’t start because of insufficient or too much priming. In my experience, many pilots only know the “if it doesn’t start, prime more” and then flood the whole engine compartment with fuel. I’ve been there, too. It is probably very difficult to set a cold engine in cold weather on fire due to too much priming but when trying to start a hot engine and continuing to prime when it wouldn’t start, is a good way of achieving that.
The C172SP (fuel injected Lycoming IO-320) has some trouble with this. Therefore, the start procedure involves turning on the electric fuel pump until there is a fuel flow indication, then pull the mixture to idle, crank the engine and once it fires, push back in the mixture. The idea is that with the mixture on idle, a potential fire would be limited to the fuel that was injected.
Unfortunately there are a million “this is the ultimate way to start an aero engine” techniques taught be the pundits but none of them work with all engines. When I used to rent out my C172M, 8 out of 10 renters were unable to start it with a hot engine and I had to intervene when fuel was dripping out of the cowling…
I have seen an aircraft badly burned by over priming. When it is worse is on a cold day with a wet tarmac. The fuel sits on top of any standing water puddles and a back fire sets it off. As a result, if I over prime the engine, I move it so the fuel is not near the exhaust pipe. The intake manifold should have a bottom drain, but these can get plugged and are rarely checked to see if they are clear. This sets up a very dangerous situation if an over prime doesn’t drain on to the tarmac.
As a result, if I over prime the engine, I move it so the fuel is not near the exhaust pipe.
How do you do that with a Cessna 404 with two pilots and nine passengers on board? I have around 2000 hours on 4-series twin Cessnas and during that time have made quite a few puddles of fuel on the tarmac. Especially in cold weather, where you usually have a single attempt at starting the first engine on battery power. Not once has that fuel on the ground ignited.
Unfortunately there are a million “this is the ultimate way to start an aero engine” techniques taught be the pundits but none of them work with all engines.
This is correct. But as said, unless there is a specifc procedure for a specific engine, the “rather more fuel than necessary” way usually works quite well.
It is sometimes very difficult to determine whether an engine wouldn’t start because of insufficient or too much priming
Agreed. I have 8 or 9 years on a number of PA28’s Archer II’s and I still dont know if I have overprimed, My technique is to run the fuel pump during pre-start / walkaround checks and make sure there is some fuel being pumped into the engine. Then prime about 4 or 5 times (on the basis that the first few pumps arent actually putting any fuel into the engine – maybe I am wrong). Then just before starting I do a couple of pumps on the throttle.
This 9/10 works for me and the engine will start within 7 or 8 turns of the prop. Have I over or under-primed? No idea, though I wish I did. As people have said, there is no “one way” to start these old planes. In case of over priming / engine fire I dont put seatbelts on or close/latch the door until the engine is running. That seems a sensible thing to do anyhow…
One mistake I’ve seen quite a bit (and done myself!) is to push the primer in too quickly. You need to let the thing fill up with fuel before pushing. If done that way, I have never needed more than 2-3 ‘primes’ to start a C172 or C182. When hot, I usually don’t prime at all on either of the above.
That said, each installation has its quirks. I remember flying a C182 Peterson with an IO470 260 HP engine some years ago, and this needed a rather special procedure to get going.
